Repositionable pressure-sensitive adhesives, adhesives which predictably adhere to, yet remain repeatedly peelable from, a variety of target substrates over a long period of time without damaging or marring the substrate, have many commercial uses. For example, masking tapes, removable labels or office notes, protective films and medical tapes all must quickly adhere to metal, paper, plastics and skin, respectively, but must also peel smoothly away from these varied target substrates without leaving behind any adhesive residue on or harming the surface of a particular target substrate.
Several approaches have been explored in preparing and formulating repositionable adhesives. One means for providing a repositionable adhesive is through the reduction of the adhesive contact area and can be accomplished by the deposition of a discontinuous or patterned film on a backing. PCT International Appl. WO 85/04602 (Newing et al.) describes pressure-sensitive adhesive articles comprising a plurality of discontinuous adhesive segments in a pattern on at least a portion of at least one side of a carrier or backing, covering from about ten to about thirty percent of the total surface area of that carrier material. These segments have an average height of from about 15 to about 35 microns and are about 50 to about 400 microns in width. The pressure-sensitive adhesive coating used must have, according to American Society of Testing Materials (ASTM) D-3330-81, a 180.degree. peel of from about 0.5 to about 2.0 pound per inch (8.75 to 35 N/dm) when such adhesive is coated and evaluated as specified by this standard. Finally, the teachings of Newing et at. explicitly state that, " . . . running together of the applied adhesive is to be avoided at all times . . . ", as such a coalescence or coating continuity will hinder the repositionability of these adhesives.
U.S. Pat. No. 4,587,152 (Gleichenhagen et at.) describes a redetachable contact-adhesive sheet-like structure prepared by the printing of a regular discontinuous pattern of calotte-shaped (cap of sphere shaped) bonding sites up to 600 microns in diameter at their base on a backing or carrier, such bonding sites comprising an adhesive having a sufficiently high structural viscosity and thixotropy to maintain their calotte shape. Gleichenhagen et al. also teach that the adhesive properties of the claimed redetachable sheet can be altered through the variation of the height, the geometrical distribution, the frequency, and the basal diameter of the calottes. It is further asserted that the adhesive properties may be varied through controlling the viscoelastic properties of the adhesive used (i.e., adhesive ranging, " . . . from very soft, highly tacky and of low shearing strength to hard, slightly tacky and of high shearing strength."). These rheological properties may be further enhanced or controlled through crosslinking the adhesive by heat or irradiation.
U.S. Pat. No. 5,194,299 (Fry) describes a repositionable pressure-sensitive sheet material comprising a sheet material bearing on one surface a discontinuous non-repetitive adhesive coating covering about 10 to about 85 percent of the surface in the form of individual adhesive islands. These islands, applied via spray coating techniques, range from about 10 to about 150 microns in height and from about 20 to about 500 microns in diameter at their bases and are comprised of a pressure-sensitive adhesive composition that, when coated continuously to a sheet material by conventional means, would not remove cleanly from a paper substrate or adherend. Fry also recognizes that the peel characteristics of the claimed sheet material may be varied by controlling the population density of the adhesive islands in the discontinuous coatings and/or the inherent tackiness of the adhesive selected for spray coating.
U.S. Pat. No. 4,889,234 (Sorenson et al.) discloses a discontinuous patterned adhesive label structure in which the level of adhesion is varied according to area of adhesive coverage on the label, the pattern in which the adhesive is coated, and the full coverage adhesive characteristics of the materials used. These variables may be adjusted independently within a single label structure, resulting in the capability to design differential peel forces at specified portions of the label. Sorenson et al. teach the criticality of selecting the adhesive material useful in the claimed structures according to their 100% coverage (i.e., continuous coating) peel force, a quantity which ranges from approximately 0.7 pound per inch (12.75 N/dm) for a solvent-type removable adhesive to approximately 6 pounds per inch (105 N/dm) for a solvent-based high strength adhesive in a 90.degree. peel test from a stainless steel substrate. As a point of reference in this disclosure, pressure-sensitive materials that are removable as continuous, 100% coverage coatings, as specified by the Pressure Sensitive Tape Council, have a peel force of about 2 pounds per inch (35 N/dm) or less.
Yet another approach to providing a permanently repositionable pressure-sensitive adhesive involves the use of crosslinking of a continuous, planer coating to reduce the tack and control the wetting or flow of the adhesive over the long term. U.S. Pat. No. 4,599,265 (Esmay) discloses a low tack, acrylate, removable pressure-sensitive adhesive tape which maintains peelability from a variety of ordinary target substrates. Esmay teaches that through the photocrosslinking of the tape's adhesive layer and the use of low levels of polar monomer (up to 3% by weight of a strongly polar monomer, such as acrylic acid) along with alkyl acrylates having side chains 8-12 carbons in length in the copolymeric adhesive, the required balance of low tack, minimal adhesion buildup, and high cohesive strength can be imparted to the removable adhesive.
U.S. Pat. No. 4,693,935 (Mazurek) discloses a continuous pressure-sensitive adhesive coating composition comprising a copolymer having a vinyl polymeric backbone having grafted thereto polysiloxane moieties. An exposed surface of the PSA coating is initially positionable on a target substrate to which it will be adhered to but, once adhered, builds adhesion to form a strong bond.
European Patent Appl. 279,579 B1 (Tanuma et al.) describes pressure-sensitive adhesive sheets comprising, in one embodiment, a continuous adhesive layer "having a macroscopically non-uniform adhesion face". These pressure-sensitive adhesive constructions, formulated to exhibit both initial and long term repositionability on a variety of target substrates, attain these removable characteristics through a combination of the partial contact between adhesive layer and adherend caused by this uneven adhesive layer and through the introduction of a crosslinking structure to the adhesive to limit the adhesion build up resulting from the fluidity or flow of the adhesive over the long term. The uneven adhesive layer, according to the application, is imparted through a variety of pressing, molding, and embossing methods.